I want to remove those buildings in order to put my own buildings there. I would like to know what i would have to do step by step to remove them, or if it is a better idea to just ask the author who made it to create a version without them.

There is one other possibility. right-click the view and select "draw game entities" and uncheck some of the stuff that is checked. But be warned, there is no guarantee it'll remove what you want removed and it may make stuff invisible you actually want in the scene.